Roadway marking systems have long been used to provide vehicular equipment operators with pertinent information through the medium of roadway markers. As example, the white stripe painted on the roadway in front of a stop sign, familiar to the lay reader, provides a vehicle operator, the driver, with a physical limit or boundary that the driver's approaching vehicle should not exceed in coming to a full and complete stop in obedience to the stop sign. On multi-lane roadways, the lanes are delineated by roadway markers. And, at major airports service roads and corridors are often distinguished, in addition to signal lamps, by painted lines marking the borders to the service road, providing a visible guide for the pilot. The foregoing are but a few of the most common applications.
In more recent experience, roadway markers have also been adapted as part of vehicular guidance and control systems. The information provided by the roadway markers is used to automatically issue an alarm or steer and/or position a moving vehicle. Sensors on the vehicle detect a marked path along a roadway and the associated control equipment on the vehicle is able to automatically correct the vehicle's steering should the sensor detect the vehicle's departure from the marked path. From time to time newspapers report of experimental automobile control systems that are intended to automatically control and guide a vehicle's travel along a highway, eliminating the need for the driver's complete attention.
Examples of the foregoing appear in the patent literature. The system in U.S. Pat. No. 5,202,742 to Frank et al carries a laser radar carried on the vehicle to detect reflective markers along a roadway. The laser beam is scanned over the roadway and the associated detectors, which receive light reflected from the roadway markers, are used by associated control equipment on board the vehicle to guide the vehicle relative to the roadway markers. Another system is presented in U.S. Pat. No. 4,947,094 to Dyer et al. In the Dyer system a linear charge coupled device (CCD) carried by an industrial warehouse vehicle, such as a forklift, monitors the position of a track, such as formed by a painted line on the warehouse ceiling overlying the roadway. The CCD images that line and that imaging allows the control equipment in the system to steer the industrial vehicle along the track.
Though painted white stripes are often used, the better roadway markers are formed of a thermoplastic material, supplied by the manufacturer as minute plastic granules or beads, that is heated to place the material into the liquid form, which can flow. Often small spheroidal glass particles are mixed into the ingredients as part of the liquid. That hot liquid is coated or extruded in a thin strip onto the roadway surface, where the plastic material is allowed to cure, that is, solidify and harden. The plastic material is designed to seep into the rough surface and pores characteristic of pavement materials, such as cement and asphalt, and hardens to form a firm grip or bond to the pavement.
Such marker is relatively wear resistant, enduring the heavy pounding and friction of automobile tires. It resists the effects of snow and rain. It also resists to the deleterious effects of sunlight, including that from ultra-violet radiation. And it maintains its color for years, ensuring a visible contrast with the surface of the roadway. Although those who apply the marker to the roadway refer to the thermoplastic film simply as "paint", an analogy to house paint, a reference that carries forward in the subsequent description, roadway markers are understood as a serious field of endeavor.
It is noted that the exact composition of the various thermoplastic ingredient materials suitable for pavement marker application, not known to the present applicants, is well known to those skilled in the road marker art. As becomes apparent those details are not necessary to an understanding of the present invention and, hence, need not be further described. Those interested in learning more on that subject, may make reference to the technical literature in that field.
The foregoing marker and control systems make use of reflected light, that is, the visible region of the electromagnetic energy spectrum, and that light originates either naturally in the environment or is generated by a light source in the detection system. Other forms of energy, though not perceptible directly by human senses, are known and have also been applied in detection schemes. As one finds from the scientific literature, the electromagnetic energy spectrum extends over a wide range of wavelengths, extending at least from the shortest wavelengths, those in the ultra-violet region and below, to and beyond the longest wavelengths in the infra-red regions. One finds visible light in this spectrum, a region which human eyes are able to detect and which enables our vision, and also radio waves. The microwave spectrum lies in a portion of that radio spectrum; and in an end portion of that microwave spectrum, one finds the millimeter wave region.
Microwave and millimeter wave energy is emitted naturally from all objects. It is also incident on our Earth from outer space, and from the Earth's atmosphere, a gaseous object, irradiating, among other things, the roadways on which we travel. Since outer space is very cold, approximately four degrees Kelvin, and since the amount of energy emitted is proportional to the emitting objects physical temperature, very little energy is incident from outer space. For the most part the incident microwave/millimeter wave energy incident on the roadway is from the atmosphere itself, which serves or acts roughly speaking like a forty degree Kelvin emitter at a 94 GHz frequency. This energy, in part, is reflected from the materials on which it is incident, including the roadway and markers on the roadway. Those materials also emit a like kind of energy, and, since those materials are typically at "room temperature", 300 degrees Kelvin, they appear warmer, higher in temperature, than those materials that principally reflect the "cold sky".
In fact, according to Planck's radiation law, any perfectly absorbing body emits radiation at all frequencies of the energy spectrum. For most natural objects in our environment, such radiation is relatively high in the infra-red region of the energy spectrum, proportional to the fourth power of the object's physical temperature. At microwave/millimeter wave frequencies, the energy is much less, varying only directly with the temperature. Though less intense, that microwave/millimeter wave energy is detectable and measurable with properly designed microwave/millimeter wave radiometers.
Microwave/millimeter wave radiometric detectors are microwave/millimeter wave receivers that detect the total power received. A microwave/millimeter wave radiometer is, in effect, a highly sensitive total power receiver. The receiver receives its signals from a directional antenna, such as a microwave/millimeter wave horn antenna, whose receiving "footprint" or field of view is directed at the element or area to be observed. The magnitude of the signal received by the radiometer is proportional to the temperature of the object under observation, and/or the temperature reflected by the object, depending upon the percentage and types of objects within the antennas footprint and the object's emissivity .epsilon., the latter being equal to (1-.rho.), where .rho. is the object's reflectivity. Radio astronomers have long used radiometric detectors to scan the heavens to detect planetary bodies and stars.
For convenience, the term, microwave/millimeter wave, is hereafter sometimes abbreviated to MMW. Thus, when used to modify the term radiometer, the abbreviated term distinguishes the radiometer discussed in connection with the present invention from other known types of radiometers, such as infra-red radiometers.
Radio astronomers also earlier determined the existence of "propagation windows" through the atmosphere for millimeter wave energy, frequencies in the 30 to 300 GHz range, at which the attenuation is relatively modest in both clear air and fog. That is, transmission of millimeter wave energy from outer space at those "window" frequencies are not attenuated in propagating through overlying clouds to their ground based radio telescopes as greatly as adjacent higher or lower frequencies in the millimeter wave region about that frequency. Similarly, the atmosphere does not emit as much energy in these windows, and, therefor does not "wash out" or overpower signals from space. These windows occur at 35 GHz, 94 GHz, 140 GHz and 220 GHz.
Imaging of ground objects, including aircraft runways, using MMW radiometric energy and MMW radiometric detectors, was proposed in the past. U.S. Pat. No. 3,725,930 to Caruso uses a microwave radiometer to detect a pattern of radiometric energy reflecting markers on an airport runway. Metal, such as iron, is known as a good MMW radiometric reflector; it is of high .rho.. In Caruso's system, wedge shaped metal plates, of a size between two feet and twenty feet in diameter, are placed on the runway as markers to provide a surface tilted up from the runway surface as presents a ramp to the oncoming airplane. Microwave energy from a portion of the cold sky is thereby reflected toward radiometric detectors carried on the taxiing aircraft, and, thus, such markers stand out from the surrounding landscape as cold looking.
Although Caruso does not expressly describe the height of the metal ramps thus employed, one appreciates that any protrusion from the roadway surface poses an impediment to the taxiing airliner and other airport vehicles using the runway. At a minimum such obstacles would cause the aircraft to sustain a series of bumps, as might disturb the passengers, and, if great enough in height, would present an obstacle that would render Caruso's system impractical of application.
Placed on the runway, the large metal plate is visible to view and does not have the anonymity of a painted stripe. It can easily be removed from the roadway surface. The metal plate thus provides an attraction to those pranksters or unscrupulous persons who would detach and sell the metal as scrap, disabling a key element of the marker system. That visibility is an unfortunate practical drawback.
Further, in an article published by the assignee of the present invention, "Passive Millimeter-Wave Imaging", Yujiri et al, Quest Magazine, TRW, Inc. 1990/1991, Vol. 13, No. 2, the authors, including one of the present inventors, present two dimensional pictures of the radiometric image of an airport, harbor and other scenes taken with experimental apparatus containing radiometric detectors tuned to receive energy at a frequency of 94 GHz. Though the images are of low resolution, the viability of radiometric inspection is demonstrated as technically feasible and urged as a soon-to-be practical means to achieve images of scenes, despite rain and fog weather conditions. The article predicts additional technical development, and, though offering suggestions for radiometric imaging, the article offers no guidance on improving roadway marker systems.
Accordingly, an object of the present invention is to provide a new method for detecting roadway markers, one that uses MMW radiometers;
It is another object of the invention to provide a roadway marker detection and control system that is passive and does not require transmitting apparatus that emits electromagnetic energy;
It is a further object of the invention to provide a roadway marker paint having enhanced contrast with the adjoining roadway surface at MMW radiometric frequencies, a true "microwave/millimeter wave radiometric" paint; and
It is a still further object of the invention to provide an enhanced radiometric paint that may be applied to roadway surfaces using existing unmodified paint striping equipment.